1. Field of the Invention
The present invention relates to dispersion stabilizers to be used for suspension polymerization or emulsion polymerization. More particularly, the present invention relates to dispersion stabilizers having excellent polymerization stability when being used for suspension polymerization or emulsion polymerization of a vinyl compound.
2. Description of the Related Art
Conventionally, polyvinyl alcohol polymers are used widely as dispersion stabilizers for suspension polymerization and emulsion polymerization of vinyl compounds. Particularly, they are useful for suspension polymerization for a polyvinyl chloride resin (hereinafter, may be abbreviated as “PVC resin”).
Since a PVC resin is excellent not only in, for example, chemical resistance and electrical insulation properties but also in processability and can be either hard or soft, it is used as various molding materials in a wide range of applications.
Generally, the PVC resin is manufactured on an industrial scale by a suspension polymerization method in which monomers of, for example, vinyl chloride (hereinafter, a vinyl chloride type monomer may be abbreviated as vinyl chloride) are polymerized in the presence of a dispersion stabilizer in an aqueous medium using an oil soluble polymerization initiator.
Examples of the factors that control the quality of the PVC resin when the PVC resin is manufactured by the suspension polymerization method include the rate of polymerization of the PVC resin, the ratio between water and vinyl chloride in a polymerization reaction system, polymerization temperature, the type and amount of polymerization initiator, the type of polymerization tank, the rate of stirring of a polymerization reaction solution, and the type of dispersion stabilizer. Among these, the type of dispersion stabilizer has a considerable effect.
In many cases, polyvinyl alcohol (hereinafter, may be abbreviated as “PVA”) is used preferentially as a dispersion stabilizer that is used for suspension polymerization of vinyl chloride (Patent Documents 1 to 4). For the purpose of improving the stability (polymerization stability) during polymerization of vinyl chloride, it has been proposed to use heat-treated PVA for a polymerization reaction (Patent Documents 1 to 3). However, when those conventional dispersion stabilizers are used to carry out the suspension polymerization of vinyl chloride, a satisfactory effect is not always obtained in terms of the polymerization stability.    [Patent Document 1] JP 51(1976)-45189 A    [Patent Document 2] JP 10(1998)-67806 A    [Patent Document 3] JP 2004-250695 A    [Patent Document 4] JP 8(1996)-259609 A
On the other hand, PVA also is heavily-used as a dispersant for emulsion polymerization of unsaturated monomers. PVAs used in general are so called “completely saponified PVA” with a saponification degree of about 98 mol % and “partially saponified PVA” with a saponification degree of about 88 mol %. In the case where the former is used as the dispersion stabilizer, a comparatively good unsaturated polymer is obtained in terms of water resistance and fluidity (high-speed coating properties). However, since the surface activity of the dispersion stabilizer itself is low, there are problems in that the viscosity of an aqueous medium increases at low temperatures and gelation occurs that makes it difficult to perform emulsion polymerization itself. On the other hand, in the case where the latter is used as the dispersion stabilizer, although the problems are solved including an increase in viscosity at low temperatures of the aqueous medium and occurrence of gelation, there is a problem in that the resultant unsaturated polymer is inferior in terms of water resistance. In order to solve such problems, both PVAs are used in combination or a PVA with a saponification degree inbetween the two is used. In the current situation, however, satisfying both the stability during polymerization and the water resistance of an unsaturated polymer at the same time has not yet been achieved.